We studied the dynamics of in vitro proliferation of tight junction strands in rat small intestine epithelia. The structure of the strands is reanalyzed in our preparations and in published micrographs. Our aim is to advance further a general model for the structure of the tight junction that explains present morphological data and accounts for permeability characteristics of diverse epithelia. The work is both experimental and analytical. To this end we induce the massive in vitro assembly of junctional strands by incubation of excised tissue (rat prostate, rat small intestine, toad bladder) in a variety of buffers at 37 degrees C. The tissues are then freeze-fractured, replicated and examined. Detailed morphological analysis includes the examination of stereo-pairs and comparison of the strands of tissues exposed to conditions that may lead to alteration of the strand itself (lipid perturbers, solvents, chelating agents). Results from the extensive literature on junctions are also being examined in detail.